It is known that some random access non-volatile memory devices can store data by altering the resistance of the memory cells therein. Such devices are commonly referred to as resistive random access memories (ReRam). In operation, a ReRam memory cell can be programmed by changing the resistance of the cell. For example, a logical data value of zero can be programmed by changing the resistance of the cell to a relatively low value, whereas as a logical data value of one can be programmed by changing the resistance of the cell to a relatively high value.
One type of ReRam is a magnetic random access memory (MRAM), which combines semi-conductor electronics and magnetics. In MRAMs, the spin of an electron, rather than the charge, can be used to indicate whether the data stored in the cell is a logical data value of one or zero.
One type of architecture used in MRAMs provides conductive lines that extend perpendicular to one another so that the conductive lines intersect with one another (sometimes referred to as a cross-point arrangement). The cells used to store data are positioned at the intersections of the perpendicular conductive lines and can be configured as a magnetic tunnel junction (MTJ) device that is accessed using an access transistor.
Data can be stored in a cell of the cross-point MRAM by generating a current in each of the conductive lines that intersect at the data cell. In particular, each of the currents flowing in the intersecting conductive lines can generate respective magnetic fields which, when combined, can affect the alignment of the magnetic moment provided by the MTJ, which can alter the resistance of the cell. For example, a first combination of magnetic fields generated by the intersecting currents can orient the magnetic moment in a first direction so that the resistance offered by the cell corresponds to a logical data value of 0. In contrast, a second combination of magnetic fields can generate an opposing magnetic moment so that the resistance of the cell is altered to indicate a logical data value of 1. Accordingly, data can be written to the cells of the MRAM by causing currents to flow in intersecting conductive lines to change the resistance offered by the cell when accessed.
FIG. 1 illustrates an equivalent circuit including a conventional cross-point MRAM where data cells are located at intersections of wordlines (WL1-3) and bitlines (BL1-4). According to FIG. 1, a data cell Cs located at the intersection of BL2 and WL2 can be written by generating respective currents IWL and IBL. The currents IWL and IBL both generate respective magnetic fields (the “hard” magnetic field and the “easy” magnetic field) in the data cell to be written. The particular combination of the magnitudes and directions of the Hhard and Heasy magnetic fields can cause the resistance of the data cell to be altered. The directions of the magnetic fields Hhard and Heasy, are based on the directions of the currents IWL and IBL.
Furthermore, ideally the magnetic fields generated by the current IWL at the remaining intersections (BL1, 3, and 4) are insufficient, by themselves, to alter the resistance of those remaining cells. It is desirable to use the combined affect of the easy and hard magnetic fields on the data cell so that the write operation to data cell Cs may be achieved. In other words, FIG. 1 shows that a magnetic fields Hhard is generated in the remaining cells due to the current IWL even those remaining cells are not selected for programming. If the magnetic field Hhard for the unselected memory cells were sufficient to change the state of the unselected data cells, the data stored therein may be unintentionally modified during the write of the selected data cell Cs.
FIG. 2 shows a range of asteroidal graphs indicating variations in magnetic fields that can affect the resistance of different MRAM cells due to process variations in manufacturing the MRAMs. In particular, FIG. 2 illustrates the different possible combined magnetic fields needed to program data to a particular MRAM data cell. As shown in FIG. 2, a first asteroidal curve AC1 indicates that a first MRAM data cell can be programmed by any combination of the Hhard and Heasy magnetic fields on the curve. It will be understood that the terms Hhard and Heasy refer to the magnetic fields generated in the long and short directions of the data cell, respectively. The asteroidal curve AC2 is shifted to the right relative to the asteroidal curve AC1 and represents a second MRAM data cell which (because of process variation) is programmed according to different Hhard and Heasy magnetic fields. Accordingly, in order to ensure that data can be programmed to any of the cells in the MRAM represented in FIG. 2, the Hhard and Heasy magnetic fields applied should be in the area referred to as “Write Margin” in FIG. 2. In other words, because of process variations, a worse case assumption may be made regarding the Hhard and Heasy magnetic fields that may be needed to program data. Therefore, as shown in FIG. 2, if the asteroidal curve AC2 reflects a “worst case” operation for a data cell in the MRAM, the MRAM operates with a relatively narrow write margin.
Although both the Hhard and Heasy magnetic fields are usually applied to a data cell in order to accomplish a write operation, it is possible to program a data cell using only one of these magnetic fields. For example, as shown in FIG. 2, the first asteroidal curve AC1 shows that if the corresponding data cell is written with, for example, an easy magnetic field that exceeds He′, the state of the data cell may be changed without any contribution of the hard magnetic field Hh′.
FIG. 3 is an equivalent diagram showing, what is referred to as a simultaneous write operation. In particular, a group of data cells Cs can be programmed by applying a current IWL to WL2 and currents IBL1-4 to bitlines BL1-BL4. As shown in FIG. 3, the combination of the respected easy and hard magnetic fields generated for each of the programmed data cells included in Cs operates to change the resistance of the data cells Cs. As shown in FIG. 4, this type of simultaneous write operation can provide for additional write margin as the same hard magnetic field is provided to each of the commonly selected data cells included in Cs.
Once data is programmed to the MRAM, the data may be read through biasing of selected data cells so that the respective resistances of those data cells may be evaluated to determine the data stored therein. In particular, different bias voltages may be applied across data cells (using the respective bitlines and wordlines/digitlines) to cause a current to flow to/from the selected data cell. The associated resistance of the data cell can be determined based on the generated current.
The structure and operation of magnetic random access memories is also discussed in, for example, U.S. Pat. No. 6,839,269 to Iwata et al. and U.S. Pat. No. 6,504,751 to Poechmueller.